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An alien species has built Dyson swarms around stars which have orbiting planets with primitive life on them. The section of the star which is facing the planet with life is left uncovered to let the planet receive enough light but the rest of star is surrounded by a Dyson swarm in the shape of some sort of creature of their choosing.

The swarm can be up to 10 times the diameter of the star in any direction and for this question we can base it on our sun and visibility from Earth.

How visible would this Dyson swarm "creature" be on the planet at any time of the day (sun rise and set may be better as there is less glare)?

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The section of the star which is facing the planet with life is left uncovered to let the planet receive enough light

This is slightly more awkward than you might think, on account of the elements of the swarm orbiting the sun at a different rate to the planet (because their orbital radii are so different).

On Earth you can create things like sun-synchronous orbits which [precess] 2 with a period of a year, but that relies on the Earth being a slightly squashed shape and the Sun is just too round to be able to pull tricks like that.

This means you'll either have to have quite a sparse swarm (eg. wide orbital radius, wide spacing between elements) or the elements will need a way to let light past when they'd otherwise shade the planet.

the rest of star is surrounded by a Dyson swarm in the shape of some sort of creature of their choosing.

This will definitely need the elements to have some kind of ability to change their habit of reflecting/occluding light. As the swarm elements transit the sun, they either need to rotate their collecting surfaces so they're parallel to the light rays directed at the planet, or they need to render them somehow transparent (like venetian blinds, perhaps) so they let enough sunlight past.

Whatever trick they use, the same approach can be used to modulate the silhouette of the swarm (by selectively letting some starlight past, or by deliberately rotating the collectors away from the sun to block starlight directed at the planet) or to deliberately reflect additional sunlight towards the planet.

How visible would this Dyson swarm "creature" be on the planet at any time of the day

Elements that appear to be close to the sun but not actually occluding it will be more or less invisible, most of the time. Special circumstances like solar eclipses by a suitable nearby body that doesn't also wholly occlude the swarm would work very well (and a monster head that appeared during a solar eclipse would certainly be one for the history books) but general observation will have to wait for the invention of the coronagraph, which is not an easy thing to discover.

After sunset, or before sunrise, a wide enough swarm can be as visible as its creators want. Light can be directed towards the planet, which is probably the most effective way to announce its presence, and a large enough and dense enough swarm will block out stars in a particular region of the sky.

There's a tradeoff to be made between swarm density and keeping the engineering difficulties of keeping the planet illuminated and a fancy shape visible, so let me suggest something rather different.

The oft-mentioned Nicoll-Dyson Laser is generally brought up as a means to blast people at stupendous distances, but it could also be used to illuminate a planet. Basically you fit an optical phased array laser to the non-collecting side of each swarm element, that lets you emit beams of light of varying powers and directions (and depending on how your array works and your tech level) different frequencies, too.

This would let you have a sun of almost arbitrary shape and colour, because what the planet sees isn't a star at all but in fact a stellar-scale lightshow.

Just the shape of a an animal? Pah, think bigger! Imagine the sun appearing as an actual animated face! Think of the fun you could have.

Any by way of a bonus you can smite your enemies even if they're a million lightyears away. What's not to like?

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Leave a gap, the sun could look like a (tilted) rectangle

The Dyson swarm will be a 2 part construct, north and south

The swarm resides inside Earth's orbit: 10x diameter of the sun, which is about 1/10 AE.

Suppose this swarm is dense. It will be built to catch a lot of sunlight, so it does.

The Earth is in orbit and you want it to receive sunlight.

So you'd have to leave a gap in the swarm, Sun can lighten up Earth in its equatorial surface.

enter image description here

A rectangular sun

The opening could be narrow. Some spreadsheet-mentality alien could have calculated how much sunshine planet Earth really needs. In that case, the opening in the Dyson swarm could be so narrow it affects the shape of the Sun.

Note about more than 1 planet

If you want to support more than 1 planet they must all orbit in the same plane. The opening in the Dyson swarm is designed to shed light on Earth's orbit plane only. Any other planet outside that plane will receive sunlight only 2x per year.

The usual night sky, spectacular sunset and sundawn

No difference at night. When your dyson construct of 1/10 AU diameter is below the horizon, the sky will look as usual. There is no difference in the night sky.

Sunset and sundawn could be spectacular.. there would be a ring around the sun, you'd see the opening in the Dyson swarm.

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  • $\begingroup$ I know there was no "science-based" tag, but I just wanted to point out that such a gap is not really reconcilable with standart orbital mechanics. If you really want such a gap the swarm needs to be fully self-propelled, which would decrease the efficiency drastically $\endgroup$ Mar 26, 2022 at 21:31
  • $\begingroup$ @AnonymousAnonymous i assume the swarm behaves organically, its components may have separate propulsion to correct position, the orbit not being a "natural" orbit. It would need to leave a gap (some way) the opening requires it. Else your sun would remain enclosed and the planet won't get sunlight. You propose a hole rotating along with the planet ? $\endgroup$
    – Goodies
    Mar 26, 2022 at 21:44
  • $\begingroup$ @AnonymousAnonymous though it wouldn't technically be a "swarm", there's nothing to stop you making a pair of statites that float over the stellar poles in the way that the illustrations in the answer show. $\endgroup$ Mar 27, 2022 at 9:36
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I would expect almost the entire creature to be invisible, except perhaps the inner surface of the opening (presumably circular) that faces the planet.

The sun is really the only significant source of light in the system. Other planets are only visible because of the sunlight they reflect. There would be no light shining on the creature's exterior. Other stars are far too distant to illuminate it at all.

The sun will appear to sit in the center of a giant starless void.

Unless the opening is much larger than necessary, the bright light from the sun will drown out the brightly lit interior back surface of the creature. The back may be so bright that the sun looks larger.

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